Atmospheric Deposition and Acid Rain

Acidic deposition, or acid rain, originates from the combustion of fossil fuels. When coal, oil, or other fossil fuels are burned, acid rain precursors--mainly nitrogen oxides (NOx) and sulfur dioxide (SO2)--are emitted into the atmosphere. Once in the atmosphere, NOx and SO2 are transformed into nitric acid and sulfuric acid and fall back to earth through both wet deposition such as rain, snow, fog, cloud water, and dry deposition of acids attached to particles, gases and aerosols. Rain and snow are somewhat naturally acidic due to the combining of carbon dioxide and water vapor in the air, which forms weak carbonic acid. However, the average acidity of rainfall in New York State is up to 30 times greater than the level typically found in rainwater.

Increased acidity has a negative effect on water quality and aquatic life. Various insects that constitute an important food source for fish-such as mayflies-are sensitive to low pH. Low pH also increases the concentration of heavy metals-such as aluminum and mercury-in the water, and can result in increased toxicity to aquatic life.

The Significance...

Low pH due to atmospheric deposition of acid rain is identified as a major source in 21% of all waterbodies assessed as impaired in New York State. However the actual impact of acid rain on the waters of New York may be somewhat greater than this figure reflects. Acid rain is more likely to affect smaller lakes and ponds, many of which are not tracked individually and/or are assessed with much larger waterbodies. The 2010 Section 303(d) List of Impaired Waters includes 72 additional lakes of less than 6.4 acres that have been identified as impaired by acid rain but that are not tracked separately in the Waterbody Inventory database.

Specific Waters...

While acid rain falls throughout New York State, many areas are less sensitive to acidity because of limestone deposits or the buffering capacity of surrounding soils which neutralize the acid. However the lack of buffering ability in the soils and bedrock of the Adirondacks, Catskills, Hudson Highlands, and Rensselaer Plateau make these areas particularly sensitive to acid rain. In fact small mountain lakes and streams of the Adirondacks and Catskills have emerged as "poster children" for the effects of acid rain.

What is Being Done...

Efforts in New York to reduce emissions which contribute to acidic deposition began in 1984 with passage of the first Acid Deposition Control Act in the nation. However even then it was clear that the state could not solve the acidic deposition problem by itself, due to the significant impact of air emissions originating primarily in the Midwest. It was reported at the time that over 80% of the sulfur deposition that occurred in the southwestern Adirondacks originated outside of New York State.

The state's early action precipitated national efforts to reduce levels of sulfur dioxide and nitrogen oxides. Title IV of the Clean Air Act of 1990 set a permanent cap on the total amount of SO2 electric utility emissions at about half the amount emitted in 1980. NOx emission-rate limitations for coal-fired electric utility units have resulted in a 27% reduction from 1990 levels.

However, in spite of these reductions, continued damage to sensitive ecosystems led New York State to require additional emissions reductions through the Acid Deposition Reduction Program (ADRP) in 2004. With the ADRP, as well as the federal Clean Air Interstate Rule aimed at control of acid rain nationwide, further reduction in acidic deposition should be forthcoming.